A level meter has an evaluation electronics system and a level sensor. The signal processing takes place in the evaluation electronics system. In the level sensors which operate according to the frequency modulated continuous wave (FMCW) transit time method or pulse transit time methods, signals, for example electromagnetic or acoustic waves, are emitted in the direction of a filling material surface. A level sensor often has an antenna for targeted transmission of the signals. After the signal is transmitted, the sensor records the reflection of the emitted signal as echo signals. Echoes may be caused by the reflections from a filling material, structures in the container interior, and the container itself. Based on the detected echo signals, which are often available as echo curves, the sensor deduces the location or position of a surface or boundary surface of at least one of the filling materials present in the container.
When acoustic or optical waves are used, the signal generated by the level meter, in particular the sensor thereof, generally freely propagates in the direction of the filling material surface to be measured. In instruments which use radar waves for measuring the filling material surface, free propagation in the direction of the medium to be measured as well as propagation inside the interior of a hollow conductor which conducts radar waves from the level meter to the medium are taken into account. In instruments which operate according to the guided microwave principle, the high-frequency signals are conducted along a waveguide toward the medium.
A portion of the arriving signals, in particular a portion of the signal energy, is reflected on the surface of the medium or bulk material to be measured, or in general on the boundary surface to be measured, and after an appropriate transit time returns to the level meter, where it may be detected by the sensor of the level meter. The unreflected signal components, in particular the unreflected component of energy contained in the signal, penetrate(s) into the medium or filling material, and propagate(s) further in the medium in the direction of the container bottom, corresponding to the physical properties of the medium. The container bottom is generally situated opposite from the level meter. The signals which penetrate into the medium are reflected on the container bottom the same way as on a boundary surface, and after passing through the medium once again, and possibly through an overlying atmosphere, return to the level meter where they may be recorded by the sensor of the level meter. The level meter receives the signals reflected at the various locations, and based on these signals determines the distance between the level meter and the filling material, according to various methods.
The book by Peter Devine entitled Radar Level Measurement: the User's Guide, ISBN 0-9538920-0-X, describes the design principle of radar level sensors.
The publication DE 10 2004 052 110 A1 describes a method for measuring a filling level in which, based on at least one property of an echo, such as a change in the transit time, a characteristic variable is determined, on the basis of which the echo is classified.
The publication DE 10 2006 019191 describes a method for determining and monitoring a filling level in a container.
The publication WO 2010/071564 A1 describes reference impedances at known positions on a waveguide in order to reflect a portion of an electromagnetic signal.
The publication US 2008/0060431 describes the determination of a filling level via a bottom reflection if the distance from the bottom is known.
The publication DE 198 24 267 A1 describes a method for recognizing useful echoes and spurious echoes, in which a measure of speed is determined for individual echoes, and the measures of speed for various echoes are compared in order to use the result of the comparison in the echo evaluation.
The publication U.S. Pat. No. 5,438,867 describes a method for measuring a level of a liquid in a container, in which a spurious signal, in particular a signal reflected on the bottom, is filtered out or otherwise taken into account by the measuring technique.
The publication WO 03/016835 describes a method for evaluating measuring signals by comparing a measurement curve to reference signal data.
The publication WO 2009/037000 describes the tracking of echoes using a tracking method.
The publication EP 2 166 320 A2 describes the use of a reflection of a higher mode for estimating the position of a reflection of a main mode.
However, none of these documents describes efficient measurement of a location of an interface.